Computational analysis via FEM of Tirefond screws used in the fastening system of railroads aiming to avoid a recurrent failure case.

Abstract

This work presents a computational evaluation of a Tirefond screw used in Brazilian railroads to support the plate on the wood sleepers. This component has been studied because nowadays it is manufactured by a common low carbon steel, and due to the increase of trains speed and the axle load transported it has been failing with a short life time, causing significant economic losses to the companies that control the Brazilian railways. In this context, static and dynamic analysis were performed using the Finite Element Method (FEM) in order to better understand the recurrent failure causes. The obtained results highlighted the critical stress concentration points where the component fatigue life is low, explaining the premature failures. Based on the generated information, it was possible to propose and to evaluate the effects of geometric and materials change on the Tirefond screw fatigue performance. The actual geometry was evaluated but replacing the actual steel by a carbonitrited SAE 1015 and by a quenched and tempered SAE 4340. Geometric changes as the increase of the screw diameter and elimination of the continuous diameter decrease as it moves away from the screw head were tested. The best solution that guaranteed a fatigue life higher than 106 cycles for all critical positions along the screw structure were the combination of the use of a SAE 4340 steel in a higher diameter screw and the use of a carbonitrited SAE 1015 steel with a continuous 22 mm diameter.